1. Field of the Invention
The present invention is mainly applied to crystal growth and post-growth process, specifically, related to a technology for growing silicon carbide single crystals by PVT (Physical Vapor Transport) and a process for in-situ annealing sublimation grown crystal.
2. Description of the Prior Art
The innovation of semiconductor technology plays an increasingly important role with the rapid development in information technologies today. The wide band-gap semiconductor materials, typically, silicon carbide and gallium nitride are the third generation of wide band-gap semiconductor following silicon and gallium arsenide. Compared with the traditional semiconductor material represented by silicon and gallium arsenide, the silicon carbide has a great advantage in properties such as operating temperature, anti-radiation, resistant to breakdown voltage. As the most developed wide band-gap semiconductor materials, silicon carbide exhibits many advantages, such as high thermal conductivity, high critical electric-field breakdown, high saturated electron drift velocity and high bonding energy etc. Its excellent performances can meet the new requirements of modern electronic technology for high-temperature, high-frequency, high-power and anti-radiation, and thus silicon carbide is considered as one of the most promising materials in the field of semiconductor materials. Furthermore, since lattice constant and thermal expansion coefficient of hexagonal silicon carbide are similar to those of gallium nitride, it is an ideal substrate material for manufacturing high-brightness light-emitting diodes (HB-LEDs).
At present, the most effective method for growing silicon carbide crystal is physical vapor transport method (Journal of Crystal Growth 43 (1978) 209-212), the typical growth chamber structure of which is shown in FIG. 1. The inner growth chamber is a graphite crucible, in which the upper part is used to glue seed crystal and the lower part is used to charge silicon carbide raw materials. Insulation is placed close proximity around, above and below the crucible which is typically graphite felt. The quartz sets of water-cooled device are located at the exterior of insulation layers. The flow rate of cooling water is required great due to radiant heat of the insulation layer. An induction coil heater is disposed around the water cooling device. Typically, silicon carbide crystal growth is performed using C plane as a growth face. The shape and the size of heat loss holes in the insulation felt can be controlled, thus silicon carbide raw materials at high temperature sublimate and decompose into vapor substances (mainly consist of Si, Si2C and SiC2), which are transported to the seed crystal at the lower temperature, and then crystallize to silicon carbide crystals. In this process, it is a key factor to obtain high-quality crystals for a suitable temperature distribution inside growth chamber. Silicon carbide crystal growth process can be generally divided into three stages, an early stage of crystal growth (i.e. seed-on stage); the early-middle crystal growth stage (i.e. diameter enlargement stage) and the mid-to late crystal growth stage (i.e. diameter growth stage). At the initial stage of single crystal growth, the axial temperature gradient inside growth chamber should be controlled relatively small so that the growth interface temperature is relatively high, and further spiral growth centers are made as few as possible, so as to achieve high-quality seed-on growth at the initial stage. At the early-middle crystal growth stage, the axial temperature gradient inside growth chamber should be controlled relatively small while the radial temperature gradient should be controlled relatively large in order to realize diameter enlargement process. At the mid-to late crystal growth stage, the axial temperature gradient inside growth chamber should be controlled relatively large while the radial temperature gradient should be controlled relatively small in order to achieve high-quality single crystal diameter growth process. In conclusion, to obtain high-quality silicon carbide crystals, it is necessary for the temperature distribution inside growth chamber to adjust in real time throughout the entire process of crystal growth. However, the present temperature field distribution inside the growth chamber is achieved mainly through designing the size and the shape of heat loss holes in the insulation materials, which is static and invariable during the growth process, thus the temperature field distribution inside the growth chamber is also static and invariable during the entire crystal growth process.
As the silicon carbide crystal growth process is carried out at a non-equilibrium state, a relative large stress exists in boules of grown crystals, which causes the crystal to rupture during the subsequent fabrication process, thereby directly reducing the crystal yield ratio. Breakage phenomenon during subsequent fabrication is particularly evident for large size crystals (3 inches or more). It is a key and urgent technical problem to optimize in-situ annealing process (annealing immediately in furnace after growth is finished) and secondary annealing process (another annealing process after taking the crystal out of the furnace) so as to remove stress in sublimation grown crystals and thereby improve the finish yield ratio of silicon carbide crystals.